Exact diagonalization of Hubbard clusters at finite temperatures
Advanced Materials Department, IPICYT, 78216 San Luis Potosí, Mexico
2 Institut für Theoretische Physik, Universität Kassel, 34132 Kassel, Germany
Corresponding author: a email@example.com
Published online: 24 January 2009
Finite temperature electronic and magnetic properties of small clusters are investigated in the framework of the Hubbard model by using exact diagonalization methods and by sampling the different cluster topologies exhaustively. Results are discussed for the specific heat C(T), magnetic susceptibility χ(T), local magnetic moments μi(T), average magnetic moments and spin-correlation functions γij(T). Representative cluster sizes and band-fillings are considered showing antiferromagnetic-like (AF) and ferromagnetic-like (FM) behaviors. For half-band filling ν= N the susceptibility shows an AF high-temperature behavior of the form χ≈1/(T + TN) from which the cluster `Néel' temperature TN is derived. In contrast, for ν= N + 1 a FM high-temperature behavior of the form χ≈1/(T - TC) is found, where TC can be interpreted as the cluster `Curie' temperature. In both cases one also observes peaks in C(T), either at T≃TN or T≃TC, which reflect the development of spin fluctuations and the breakdown of the low-temperature short-range magnetic order. The dependence of TN and TC on cluster size N and interaction strength U/t is analyzed in terms of effective Heisenberg spin interactions. Finally, the effects of temperature-induced structural fluctuations are discussed.
PACS: 36.40.Cg – Electronic and magnetic properties of clusters / 75.10.Lp – Band and itinerant models
© EDP Sciences, Società Italiana di Fisica, Springer-Verlag, 2009